156 ORIGIN OF NATURAL IMMUNITY, 



geiiit3'of these produce the condition known as natural immunity. 

 As the bacteria are constantly traversing the mucous membrane 

 and being dissolved, the stock of immune bodies is being con- 

 stantly replenished. With a perfectly sound and health}- animal 

 there will be an abundance, so to speak, of immune bodies, and 

 the bacteria will in consequence be dissolved soon after leaving 

 the lumen of the intestine. Thev will not get so far as the orgfans 

 which will be found to be sterile, so that investigators might 

 conclude that bacteria given with the food do not leave the intes- 

 tine. Should the animal not be perfectl}^ healthy and the store of 

 immune bodies low, the intestinal micro-organisms will probably 

 travel further and be found in the glands and organs. The same 

 thing would happen if the intestine were injured, for the num- 

 bers crossing the wall would be relatively enormous, and the small 

 supply of immune bodies would be quickly used up. While 

 positive results, obtained by sowing mucous membrane and 

 lymphatic organs in nutritive media, are undoubted proofs of the 

 passage of bacteria from the intestine, negative results simply 

 show the absence of living bacteria. The sterility may be due 

 to no bacteria having traversed the intestinal wall or having 

 crossed, they have been destroyed during or after the passage by 

 the immune bodies. Experimental work upon this subject will, 

 therefore, always be conflicting. 



It is thus unreasonable to expect to obtain information regard- 

 ing the permeability of the intestine to putrefactive and saprophy- 

 tic bacteria by an examination of the organs. With pathogenic 

 bacteria the case is different. If they survive the passage through 

 the acid juice of the stomach, they Avill traverse the intestinal 

 walls and, multiplying in the tissues and organs, will produce their 

 characteristic lesions, provided that they can get across the mucosa 

 in sufficient numbers to absorb any trace of specific immune body. 



I have regarded the solution or lysis of bacteria as a process of 

 digestion, all the substances which are necessary to dissolve the 

 cells being called the bacteriolytic immune bodies. Several 

 substances are involved in the lysis of one cell. Until quite 

 recently two were known — the immune body and the complement 



